1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser. More particularly, the present invention relates to a vertical cavity surface emitting laser, which is provided with an aperture for guiding a flow of electric currents, wherein the aperture is defined by an oxide so that the aperture is formed approximately in a circular shape, and a method for fabricating the laser.
2. Description of the Related Art
In the prior art, unlike an edge emitting laser, a vertical cavity surface emitting laser (VCSEL) emits a Gaussian beam that is formed approximately in a circular shape in the laminating direction of semiconductor layers, so that the VCSEL does not require an optical system for correcting the emitted shape of the light. Furthermore, because it is possible to reduce the size of a VCSEL so that a plurality of VCSELs can be integrated on a single semiconductor wafer, it is easy to array the VCSELs in two dimensions. Due to this advantage, VCSELs find wide application in the optical fields, such as the optical communication field, electronic calculators, audio/video appliances, laser printers, laser scanners, medical appliances, etc.
Such a VCSEL includes one or more insulation regions formed in an upper reflector layer in order to enhance an optical output performance by guiding a flow of electric currents delivered through an electrode. Such an insulation region may be formed by two different processes: (1) an ion implantation process, in which an upper reflector layer is formed and then protons, ions and the like are implanted; and (2) a selective oxidation process, in which peripheral areas except a current guide region are oxidized by controlling time. The ion implantation process is advantageous with regard to product yield and the readiness in the fabrication of products. However, the ion implantation process has drawbacks in that a resulting device can have a defective performance during high-speed operation, because it is difficult to control a leakage current, and in that reproducibility is poor in mass production because implanted protons are not uniformly distributed.
By comparison, in the selective oxidation process, the periphery of a plurality of surface emitting lasers grown on a single substrate is etched away in a state in which a part of a layer under an upper reflector layer, i.e., a portion intended to be formed with an insulation region, is laminated with an AlGaAs layer, and then the AlGaAs layer is diffusedly oxidated inward by controlling the length of time within an oxidation environment, as a result of which an Al2Ox oxidation insulation film, i.e. an insulation region for limiting a flow of electric currents, is formed.
FIG. 1 is a cross-sectional view which shows a structure of a VCSEL formed by the conventional selective oxidation process. FIGS. 2a to 2d are stepwise cross-sectional views which show a procedure of fabricating a VCSEL, and FIG. 3 is a photograph taken by an infrared camera, which shows the plan view of the VCSEL in the step shown in FIG. 2c. The structure of a VCSEL formed by means of a conventional selective oxidation process and the steps for fabricating the VCSEL will now be described with reference to these drawings.
As shown in FIG. 1, a VCSEL structure comprises a semiconductor substrate 10, a lower reflector layer (n-DBR: Distributed Bragg Reflector) 11, an active layer 12, an upper reflector layer (p-DBR) 14, at least one insulation region 15, an insulation film 16, at least one upper electrode 17, and a lower electrode 18.
In the fabrication process, at first, the lower reflector layer 11, the active layer 12, and the upper reflector layer 14 are laminated on the semiconductor substrate 10, as shown in FIG. 2a. Here, an oxidable layer (AlAs) 13, which will form the insulation regions 15 through a subsequent oxidation step, is intervened between the active layer 12 and the upper reflector layer 14.
Next shown in FIG. 2B, a mask pattern is formed on the upper reflector layer 14 and then a trench B is formed by mesa A etching using a dry etching process, so that the oxidable layer 13 can be exposed to an oxidation environment. If an oxidation environment is developed for a predetermined length of time after the trench B is formed, the oxidable layer 13 is oxidated inward from the parts exposed by the trench B, thus forming a plurality of insulation regions 15, C of Al2O3, as shown in FIG. 2c. Here, a selective oxidation process is carried out so that the central region of the oxidable layer 13 is not oxidated. The non-oxidated region of the oxidable layer 13 surrounded by the oxidated insulation regions 15, C is called as oxidation aperture D, by means of which the emission shape of light outputted from the VCSEL is determined.
FIG. 3 is a plan view after the selective oxidation process (the step shown in FIG. 2c), from which drawing the structure of the trench B can be seen which is formed by mesa A etching and the shapes of the insulation regions C and the oxidation aperture D can also be seen in detail.
As shown in FIG. 2d, the insulation film 16 and the upper electrodes 17 are formed, and the lower electrode 18 is formed on the lapped bottom surface of the substrate 10, thereby completing the fabrication of the VCSEL. Here, the upper electrodes 17 are formed with a window by a metal aperture E so that light produced from the active layer 12 can exit, wherein the upper electrodes 17 are independently formed for each VCSEL, so that one VCSEL can be driven independently from an adjacent VCSEL.
The VCSELs of the above structure can be used in a one-chipped array structure or can be individually used by being separated from each other.
However, the most serious problem of a VCSEL formed by the selective oxidation process is that the far field pattern (FFP) control is very difficult. The FFP control is preformed in two ways, one of which is to control the light-emitting angle and the other of which is to control the light-emitting shape. In controlling the light-emitting angle, it is possible to find out an appropriate value by controlling the sizes of the metal aperture and the oxidation aperture. Whereas, because the emitting shape is determined by the shape of the oxidation aperture, it is not easy to control the light-emitting shape. Due to the lattice directionality of a semiconductor, oxidation rates are varied even if oxidation processes are performed for a same length of time under the same conditions. Eventually, as shown in FIG. 3, the oxidation rate is faster in the diametric direction Di than either in the vertical direction V or in the horizontal direction H, as a result of which the oxidation aperture is formed in a diamond shape rather than in a circular shape, thereby bringing difficulties in controlling the light-emitting shape.